Turbo charging an engine allows the engine to provide power similar to that of a larger displacement engine while engine friction is maintained near the level of a normally aspirated engine of similar displacement. Thus, turbo charging can extend the operating region of an engine. However, during conditions where there is high flow through a compressor of the turbocharger, the compressor speed may cause increased noise, vibration, harshness (NVH), or other undesirable responses. Such noise levels may be particularly apparent during conditions where the engine operates at relatively low engine speeds, and produces relatively little noise.
For example, during cold start conditions, the engine may be controlled to increase the volumetric efficiency of the engine in order to maximize the flow rate of heated exhaust gas through an exhaust after-treatment device to more quickly heat the exhaust after-treatment device to a suitable operating temperature. By increasing the flow rate of exhaust gas and/or intake air, the speed of the turbocharger is increased and an unbalance of rotating components in the turbocharger excites the engine and exhaust structure, which results in increased noise. On the other hand, although the flow rate is increased, the engine speed remains relatively low and produces relatively little noise. Accordingly, the turbocharger is perceived as being excessively loud relative to other components of the engine during these conditions.
The inventors herein have recognized the above-mentioned issues and have developed a method for reducing noise generated by the turbocharger, comprising: opening a turbocharger bypass valve to reduce a turbocharger speed responsive to a cold start condition.
In one example, by opening the turbocharger bypass valve, air/exhaust flow is recirculated around a compressor absorbing more shaft work and thus decreasing the speed of the turbocharger. During this event the exhaust flow is unchanged and the engine continues to provide the increased level of high temperature exhaust to the aftertreatment device. The reduction in speed of the turbocharger causes the noise level of the turbocharger to be reduced to a suitable level that is more aligned with the level of noise produced by the engine at idle.
In another example, by opening the turbocharger bypass valve, the energy of the exhaust stream bypasses the turbine and it is not able to provide the work required for a high turbocharger speed. The speed reduction results in a reduction of the NVH condition.
Moreover, during a cold start condition, the likelihood of a significant acceleration request (e.g., tip-in) is low. Accordingly, throttle response penalties due to turbocharger lag as a result of reducing the turbocharger speed rarely may be realized. Further, in some embodiments, the method may include closing the turbocharger bypass valve responsive to an acceleration request being greater than an acceleration threshold. The turbocharger bypass valve is closed to increase the turbocharger speed for reduced turbocharger lag and better vehicle launch readiness. Upon vehicle launch and acceleration, the engine speed is increased, and correspondingly the engine noise level becomes loud enough that the noise generated by the turbocharger is absorbed or drowned out by the engine noise. In other words, under these conditions, the noise generated by the turbocharger is no longer perceived as being excessively loud compared to the other components of the engine.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.